Filters that can be used as receive filters must have minimal insertion loss in the pass band, must suppress the respective counter band or bands to a maximum degree and be adapted with respect to the impedance. The bandwidth of the passband is a further parameter of a filter.
For particularly broad frequency bands of e.g. over 4%, a good impedance adjustment over the entire bandwidth is very difficult to realize. Known solutions usually lead to an insertion loss that is too high and an edge steepness that is too low, which makes the suppression of the counter band difficult or even impossible, if the counter band is directly adjacent to it. For covering a particularly broad reception band, therefore e.g. two filters were used, which divided the broad band into two sub-bands that were respectively allocated to an own filter each, which filter could be selected via switches as required.
The operational mode “Carrier Aggregation”—or CA mode—is increasingly gaining importance for future mobile radio applications. It involves parallel data transmission in two different bands for increasing the bandwidth of a call connection. For Carrier Aggregation, frequency bands with a bigger frequency spacing are used in particular. This facilitates multiplexing and prevents any acoustic disruptive effects, which one filter can create in the band of the respective other filter. Thus, a simultaneous operation of both filters with good adaptation is possible. For bands that are closer to one another, it is sometimes required to use one filter with a very large bandwidth, which captures both bands, whereby, however, the separation of the two bands from each other is often very difficult to achieve.